Telegraphone and like apparatus



Nov. 9, 1937. N. M. RUST ET AL 2 8,

- TELEGRAPHONE AND LIKE APPARATUS Filed July 23, 1935 INVENTORS f/OEL MEYER 2015' 7' )E/L J06 FHA/[II C BY/ TWEED Ai'TORNEY Patented Nov. 9, 1937 UNITED STATES PATENT omen 2.098.596 TELEGBAPHONE AND uxnmm'ms Noel Meyer 1am and cyan Johnlrancis Tweed,

Essex, England, assignors to Radio Corporation of America, a corporation of Delaware Application suly'zs, 1935, Serial No. 32,152

In Great Britain May 28, 1934 6 Claims. (cl. ire-loos upon and subsequently picked up. from a moving tape or ribbon of magnetic material by means of electro-magnetic recording and pick-up devices comprising a pair of magnetic pole-tips cated on opposite sides of the tape and associated 10' with a winding or windings carrying the signal currents to berecorded or picked up, it is usual to employ pole tipsof chisel shape and it is found in practice that wi h pole-tips of usual construction considerable ear takes place by reason of 1s friction with the travelling tape.-and the useful may be not more than two to life of a pole-tip three hours before deterioration of the characteristic of the recording or pick-up apparatus as V .-a whole'occurs on account of the alterationin flux distribution due to the wearing away of the pole-tips.

As above stated the pole-tips of such electromagnetic recording or pick-up devices have hitherto generally been given a chisel-like form as shown inl lgures l, 2 and 3 of the accompanying drawing. In Figures 1, 2 and 3 the tape is indicated by the reference I and 2, and 3 are the poletips. Figures -1 and 2 are mutually perpendicular views and Figure 3 is a view looking from the left in Figure 1 and showing the face of the poletip 3. As will be seen the edge of the chisel is arranged transverse to the moving tape and the pole-tips on opposite sides of the tape are slightly staggered lengthwise thereof. Thus as wear pro- 5 ceeds the chisel-like edge of the pole-tip is worn fblunt" and the efiective contact area between vide an improved, electro-magnetic recording or pick-up device in which wear of the pole-tips is substantially reduced and wherein the adverse cf- 5 feet upon the response characteristic of such wear as takes place is reduced or eliminated whereby the useful-life of the pole-tips is greatly increased. This object is achieved according to the said invention by making the pole-tip of an I electrc-magnetic recording or pick-up device for use in a telegraphone or like recording or pickup system of substantially uniform cross-section throughout an appreciable portion of the length thereof and embedding said portion in magnetili cally and electrically inert material (1. e., insu-- lating material) offering a resistance to wear of the same order as that of the material of the pole-tip proper.

The larger wearing surface presented by the insulating material in which the pole-tip is em- 5 bedded in accordance with this invention tends to minimize wear, the cross-sectional area of the pole-tip proper being small compared with that of the material in which itis embedded. The effect of any wear which'does take place is mini- 1o mized by making the magnetically active part of 1 the-pole-tip of a thin parallel-sided lamination so that over the useful life of the tip substan tially the same contact area of magnetic material is exposed to the tape. The width of the lamina- 16 tion is, of course, arranged transversely of the c direction of movement of the tape. Experiments show that a pole-tip constructed in accordance with the invention should have a normal life of about thirty hours or more.

The insulating material at the face of the pole-. 20 tip has to be carefully chosen since if it is harder than the magnetic material a certain undercutting action is liable to take place after prolonged wear. If, on the other hand, it is too soft it does 2 not substantially increase the wearing properties of the tip. Experiments have shown that a vulcanite cement manufactured for dental work is a very suitable material, and such material can conveniently be moulded in a jig round the lamio nation constituting the magnetic portion of.the pole-tip with the aid of gentle heat.

To refer again to present common practice, it is customary in known telegraphone pick-up apparatus to arrange two chisel shaped pole pieces, one on each side of the recorded tape or wire, and in "staggered relation, 1. e., not directly opposite one another but spaced apart in the direction of record travel. The amount of stagger has an important effect upon the frequency response characteristic; maximum output at any .given frequency occurs when the stagger is equal to the length on the record of one half cycle of that frequency the said output falling away as the stagger is increased, reaching a minimum when the stagger is equal to the lengthon the record of onecycle of the frequency in question. Thus the use of a large "stagger" tends to increase response at low frequencies and reduce that at high while the use of a small stagger" has a converse effect, reducing the output at low frequencies and causing it to be distorted. Therefore the present general practice is to adopt for'the usual chisel-shaped poles of the pick-up or reproducing "head a is obviously reduced thereby. Another importantj compromise stagger such that, in the case of a record which has been recorded'by a sir-called constant current method; the reproducer output frequency curve has a peak at about 2,000 cycles per second and falls away gradually on either side of this peak. It is a comparatively object of this invention is to overcome this difficulty or defect and this object is achieved according-to the invention by replacing the hitherto customary chisel shaped poles by blunt pole pieces in staggered relationship. These pole pieces may be, and preferably are, laminated to reduce losses and are arranged to be in good mechanical contact with the record over substantially their whole area. This may conveniently be effected by mounting each pole piece with its active end or face flush with the surface of end cheek of hard insulating material, such as ivory.

The invention is illustrated in Figures 4 to 18 inclusive of the accompanying drawing.

Figures 4, 5, and6 show one embodiment of the invention, Figures 4 and 5 being'mutually perpendicular views and Figure 6 showing the face of the pole piece 3 as viewed from the left of Figure 4. Referring to' Figures 4, 5 and 6 each of the pole-pieces 2, .3, of the recording or pickup device are built up from a number of laminations 2L or 3L of magnetic material the centre lamination in each case projecting beyond the 'laminations on either side thereof to constitute the pole-tip, the projecting end of said centre lamination being slightly reduced in width as shown in dotted lines in Figure 5 and embedded in insulating material 2| or ii to bring the overall dimensions of the composite working face up to those-of the cross-section of the pole pieces.

In order to take up wear and to allow for any irregularity in the movement of the tape, the

two oppositely located pole pieces are mounted in a manner so as to be relatively movable and in one arrangement illustrated in the accompanying Figures 7, 8 and 9 (which are mutually perpendicular views, Figure 8 being half in section) the laminated pole-pieces 2. 3, are slidably arranged in a laminated yoke l of magnetic material adapted to link the two pole-pieces together, the pole-pieces being lightly loaded by springs (not shown) so as to urge the pole-tips into contact with the moving tape I. In such an arrangement the magnetic yoke, whichis approximately c-shaped may form with the pole-pieces a substantially complete magnetic circuit except for the gap through which the moving tape passes, or, as illustrated, the yoke may be split into two halves at a point 5 opposite the gap for the tape each half of the laminated yoke being clamped in aninsulating block (not shown) and the said blocks hinged about suitable axes e. g.,

axes passing through the crosses marked 8, I, in Figure 8, to facilitate inserting and withdrawing the tape. The windings I, 9, are arranged on bobbins II, I! surrounding the pole-pieces, said bobbins being attached to the yoke and the polepieces being free to slide in said bobbins.

In an alternative arrangement illustrated in the accompanying Figures 10, 11 and 12 (which again are mutually perpendicular views) each of the laminated pole pieces 2, I, constitutes the middle limb of an E-shaped magnetic circuit [3, ll; the outer limbs of which constitute the return magnetic paths. These two E-shaped magnetic circuits are located on opposite sides of the tape I and a slight air-gap I5 is left between the outer limbs of the oppositely disposedE-shaped circuits. Each of the magnetic circuits is urged towards the tape by means of springs (not shown), each circuit, including its pole piece,

' moving as a whole. This arrangement has a good magnetic circuit since there are double return paths provided by the outer limbs of the E-shaped circuits and since the laminated polepieces are clamped to their respective cores there is no relative movement due to irregularities in the tape and thus fluctuations in the reluctance of the magnetic path due to such movement are eliminated, the only change in the reluctance of the vmagnetic circuit being due to a comparatively very small change in the air-gaps. Movement of the pole-pieces relative to the core tends to cause his and crackles in the reproduced signals.

In the arrangement of Figures 10 to 12 the windings l, 8, are fixed relatively to the polepieces and this avoids any contributory cause of hiss which might be caused by' slight relative movement between the energizing windings and the pole-pieces.

It will be noted that in the embodiments illustrated so far the return magnetic paths for flux traversing the pole-pieces, leads the said return flux away from the tape and thus substantially and stray induction effects from outside fields- Figures 13, 14 and 15 show another embodiment of'the invention as applied to a reproducer or pick-up, Figures 13 and 14 being mutually perpendicular views and Figure 15 showing the face of the pole piece 3. Each pole piece is preferably'laminated, as shown, and has its active face in contact with the tape I the pole pieces being blunt and of such size as to give relatively large area pole faces in-contact with the tape. ts will be seen-the poles are staggered but, since the said poles are not chisel shaped in effect the advantages of small and large staggers" are simultaneously obtained. For high frequen- I cies the effective stagger" is the dimension b" while for low frequencies the effective stagger is the dimension "n". The thickness or, each pole piece (1. e., its dimension in the direction of tape travel) is considerably greater than the length on the tape of a half-cycle of a. high frequency and this leads to a certain amount of loss of high frequency response, but this is not a serious disadvantage (if it is a disadvantage at all) since it eliminates the double humping which would otherwise occur in the frequency response characteristic.

In order to ensure good mechanical contact between pole faces and tape over the whole pole face areas it is preferred to modify the arrangement of Figures 13, 14 and 15 as shown in Figurea 18, Hand 18. Figures 16 and 17 are views corresponding to the views of Figures 13 and 15 respectively and Figure 18 is a view taken at right angles to Figure 17 and showing the end cheek and pole arrangement. In the modification of Figures 16 to 18 each pole ismounted with its active iacetlush with the surfaceotanend cheek II or I? made of a hard insulating material such, as ivory and formed with a central raised rib Ila or Ila with the surface or which the pole face in questionis flush. The rib faces and the pole faces flush'therewith bear asainst the tape and will wear away relatively slowly and substantially evenly.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

1. An electro-magnetic recording or pick-up device comprising a pole tip made of substantially uniform cross-section throughout an appreciable portion or its length, said portion being embedded in a mouldable non-magnetic insulating material having a resistance. to wear of the same order as that of the material of the poletip proper, the said pole tip being made of laminations extending transversely of the direction of record travel. 7

2. Apparatus as claimed in claim 1 wherein the mouldable non-magnetic insulating material is vulcanite cement as employed for dental work.

3. Apparatus as claimed in claim 1 and wherein the mouldable non-magnetic insulating material is ivory.

4. Apparatus as claimed in claim 1 and wherein the pole tips form part of a laminated magnetic circuit.

5. Apparatus as claimed in claim 1 and wherein the laminated magnetic circuit is fixed in relation to the telegraphone tape or the like, said pole tips being movable in relation to said circuit and biased towards said tape or the like.

6. Apparatus as claimed in claim 1 and wherein the laminated magnetic circuit is movable and biased as awhoie towards the telegraphone tape or the like said pole tips being fixed in relation to said circuit.

NOEL MEYER RUST. CYRIL JOHN more TWEED. 

